1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly to a semi-transmissive liquid crystal display device having a reflective region used to perform display functions by reflecting light incoming from outside and a transmissive region used to perform display functions by allowing light from a light source provided on the backside of the device to transmit through the transmissive region.
2. Description of the Related Art
Conventionally, two primary types of liquid crystal display devices have been known. One of the two primary devices is a reflective liquid crystal display device which includes a reflector within the device in order to reflect light incoming from outside and serving as a light source for display by using the reflector and eliminates the need for a backlight as a light source. The other is a transmissive liquid crystal display device having a backlight provided therein as a light source.
Since the reflective liquid crystal display device eliminates the need for a backlight, which is indispensable for formation of a transmissive liquid crystal display device, the reflective device consumes lower electric power and is fabricated thinner and lighter. Accordingly, the reflective liquid crystal display device is utilized primarily as a portable terminal device. In contrast, since the transmissive liquid crystal display device has a backlight therein as a light source, the device is advantageously able to clearly display an image to be viewed even when the amount of light from the surroundings is small, i. e., the surroundings are dark.
In consideration of features found in the two primary types of liquid crystal display devices, a semi-transmissive liquid crystal display device whose cross sectional view is shown in FIG. 19 is disclosed as a liquid crystal display device that has both advantages found respectively in the reflective liquid crystal display device and the transmissive liquid crystal display device and includes both a reflective region 5 and a transmissive region 6 in one pixel (refer to Japanese Patent No. 2955277). In the disclosed semi-transmissive liquid crystal display device, light travels different distances in a liquid crystal layer respectively when entering the reflective region 5 followed by reflection by the same and when transmitting through the transmissive region 6. That is, light travels back and forth within the liquid crystal layer in the reflective region 5 and travels only one time through the liquid crystal layer in the transmissive region 6. To prevent occurrence of the difference in distances that light travels in the liquid crystal layer, the liquid crystal display device shown in FIG. 19 is configured to have an insulation layer 8 formed below a transparent electrode 7 in the reflective region 5 and dispose a reflector 9 below the insulation layer 8, causing a difference between a thickness dr of the liquid crystal layer in the reflective region 5 and a thickness df of the liquid crystal layer in the transmissive region 6. Accordingly, the difference therebetween gives solution to a problem of impossibility of optimizing the intensity of light exiting the device, which problem is due to different retardation values that both regions have.
As described above, forming the transmissive region and the reflective region in a pixel electrode makes it possible to use a liquid crystal display device as a reflective liquid crystal display device by turning off a backlight when the surroundings are bright, thereby effecting low power consumption that is to be achieved by employment of a reflective liquid crystal display device. Furthermore, in a case where a liquid crystal display device is used as a transmissive liquid crystal display device when the surroundings are dark and the backlight is turned on, the liquid crystal display device enhances the visibility of an image to be displayed when the surroundings are dark, which operation is featured in a transmissive liquid crystal display device.
A liquid crystal display device can also be grouped into two primary devices in terms of its operation. That is, one of the primary devices called a vertical electric field type is configured to perform display functions by making a liquid crystal molecule whose major axis is previously aligned in a predetermined direction (referred to as a director) rotate in a plane perpendicular to a substrate and the other called a horizontal electric field type is configured to perform display functions by making the liquid crystal molecule rotate in a plane parallel to a substrate.
A vertical electric field type transmissive liquid crystal display device has worse viewing angle characteristics as compared to a horizontal electric field type transmissive liquid crystal display device. However, in a reflective region to which a vertical electric field is applied, light incident on the region and light reflected from the region travel in directions reverse to each other relative to a direction (direction of optical axis) of the principal indices of refraction of a liquid crystal molecule, in other words, travel in a direction substantially symmetrical relative thereto. Accordingly, the amount of birefringence of the area irradiated by the light incident on the region and the amount of birefringence of the area irradiated by the light reflected from the region are cancelled each other to reduce the amount of change in the birefringence, achieving desirable viewing angle characteristics.
In order to further improve the viewing angle characteristics of the semi-transmissive liquid crystal display device, a technique that employs a transmissive region to which a horizontal electric field is applied has been proposed (Japanese Patent Application Laid-open No. 2001-042316, Japanese Patent Application Laid-open No. 2001-083494, Japanese Patent Application Laid-open No. 2001-125096, Japanese Patent Application Laid-open No. Hei 11-167109).
The inventors of the application found that when the semi-transmissive liquid crystal display device shown in FIG. 19 employs a horizontal electric field, the device operates in a normally-white mode in the reflective region 5 and in a normally-black mode in the transmissive region 6, meaning the device is far from serving as a practical usage. How the device operates will be explained in detail below with reference to the drawings.
FIGS. 20(a), 20(b) and 20(c) are schematic diagrams of the semi-transmissive liquid crystal display device shown in FIG. 19 and having therein both the reflective region 5 and the transmissive region 6 to which a horizontal electric field is applied. In particular, FIG. 20(a) illustrates how the associated components are optically arranged and FIG. 20(b) illustrates an alignment angle at which a polarizer and a liquid crystal layer are oriented relative to each other when viewing the device from the side of an opposing substrate 12, and FIG. 20(c) illustrates how the polarizer and the liquid crystal layer operate in the reflective and transmissive regions.
As shown in FIG. 20(a), a semi-transmissive liquid crystal display device 50 includes: a lower substrate 11; an opposing substrate 12; a liquid crystal layer 13 sandwiched between the two substrates; and a backlight 40 disposed below the lower substrate 11, in which the lower substrate 11 and the opposing substrate 12 have polarizers 21a and 21b provided respectively on the outer sides of the substrates. Though not shown in FIG. 20(a) for simplification, the lower substrate 11 and the opposing substrate 12 have horizontal alignment films for aligning liquid crystal molecules in a horizontal direction provided respectively on surfaces, contacting the liquid crystal layer 13, of the substrates. An angle made between the two alignment films, provided on the surfaces of the two substrates, for aligning liquid crystal molecules in a horizontal direction is referred to as a twist angle.
The lower substrate 11 has a first insulation film 8a provided on a side, facing the liquid crystal layer 13, of the substrate 11. In a reflective region 5, the lower substrate 11 has a second insulation film 8b formed on the first insulation film 8a and a reflector 9 formed on the second insulation film 8b, and then, a third insulation film 8c formed on the reflector 9, and further, an electrode 7 for generation of horizontal electric field formed on the third insulation film 8c. The electrode 7 for generation of horizontal electric field consists of a pixel electrode 27 and a common electrode 26 disposed in parallel with each other, and an electric field generated between the pixel electrode 27 and the common electrode 26 drives the liquid crystal layer 13. In a transmissive region 6, the lower substrate 11 has a pixel electrode 27 and a common electrode 26 formed on the first insulation film 8a and disposed in parallel with each other, and an electric field generated between the pixel electrode 27 and the common electrode 26 drives the liquid crystal layer 13. The second insulation film 8b and the third insulation film 8c are provided to adjust a difference between gaps formed by thicknesses of the liquid crystal layer 13 in the transmissive region 6 and the reflective region 5.
As shown in FIG. 20(b), when a voltage is not applied between the common electrode 26 and the pixel electrode 27, and an alignment angle at which the polarizer 21a located on a lower side of the reflective region 5 and the transmissive region 6 is oriented is assumed to be zero, the polarizer 21b located facing the polarizer 21a is made to have an alignment angle of 90 degrees and the liquid crystal layer 13 is made to have an alignment angle of 45 degrees.
How the semi-transmissive liquid crystal display device operates under the aforementioned conditions is shown in FIG. 20(c). The device operates in the reflective region 5 as follows. When a voltage is not applied between the pixel electrode 27 and the common electrode 26, linearly polarized light having transmitted through the polarizer 21b and having an alignment angle of 90 degrees transmits through the liquid crystal layer 13 and then becomes right circularly polarized light. Thereafter, the right circularly polarized light reaches the reflector 9 and is reflected as left circularly polarized light by the reflector 9, and again transmits through the liquid crystal layer 13 and becomes linearly polarized light having an alignment angle of 0 degrees, preventing the light from exiting the device and in turn being followed by a display of black color. When a voltage is applied between the pixel electrode 27 and the common electrode 26, the liquid crystal layer 13 changes its state and comes to have an alignment angle of 0 degrees. In this case, the linearly polarized light having transmitted through the polarizer 21b and having an alignment angle of 90 degrees keeps unchanged even after transmission through the liquid crystal layer 13. Then, the light reaches the reflector 9 and is reflected by the reflector 9, and again transmits through the liquid crystal layer 13 and exits the device while keeping its linearly polarized state and having an alignment angle of 90 degrees, leading to a display of white color. That is, the device operates in a normally-black mode in the reflective region 5.
The device operates in the transmissive region 6 as follows. When a voltage is not applied to the liquid crystal layer 13, linearly polarized light having transmitted through the polarizer 21a (and having an alignment angle of 0 degrees) transmits through the liquid crystal layer 13 and then becomes linearly polarized light having an alignment angle of 90 degrees. Thereafter, the linearly polarized light exits the polarizer 21b having an alignment angle of 90 degrees, leading to a display of white color. When a voltage is applied to the liquid crystal layer 13, the liquid crystal layer 13 changes its state and comes to have an alignment angle of 0 degrees. In this case, the linearly polarized light having transmitted through the polarizer 21a (and having an alignment angle of 0 degrees) keeps unchanged even after transmission through the liquid crystal layer 13 and then does not exit the polarizer 21b having an alignment angle of 90 degrees, leading to a display of black color. That is, the device operates in a normally-white mode in the transmissive region 6.
Subsequently, how a semi-transmissive liquid crystal display device 51 having a reflective region 5 to which a vertical electric field is applied and a transmissive region 6 to which a horizontal electric field is applied operates will be explained below. FIG. 21(a) illustrates how the associated components are optically arranged in the semi-transmissive liquid crystal display device 51 and FIG. 21(b) illustrates an alignment angle at which a polarizer and a liquid crystal layer are oriented relative to each other when viewing the device from the side of an opposing substrate 12, and FIG. 20(c) illustrates how the polarizer and the liquid crystal layer operate in the reflective and transmissive regions.
The difference between the optical arrangement applied to the reflective region 5 shown in FIG. 21(a) and the optical arrangement applied to the reflective region 5 shown in FIG. 20(a) is that the device shown in FIG. 21(a) does not have the reflector 9 and the electrode 7 for generation of horizontal electric field, those components being provided in the device shown in FIG. 20(a), and instead, has a reflecting pixel electrode 10 formed on a second insulation film 8b and an opposing electrode 14 formed on an opposing substrate 12 so as to face the reflecting pixel electrode 10. The device shown in FIG. 21(a) is configured to generate a vertical electric field between the reflecting pixel electrode 10 and the opposing electrode 14 in the reflective region 5. Note that the optical arrangement applied to the transmissive region 6 shown in FIG. 21(a) is the same as that applied to the transmissive region 6 shown in FIG. 20(a). Furthermore, when viewing the device from the side of the opposing substrate 12, a polarizer and a liquid crystal layer shown in FIG. 21(b) have the same alignment angles as those shown respectively in FIG. 21(b), and therefore, the explanation of the optical arrangement and the alignment angles shown in FIGS. 21(a), 21(b) is omitted.
How the semi-transmissive liquid crystal display device 51 constructed in the aforementioned manner operates in the reflective region 5 will be explained with reference to FIG. 21(c). When a voltage is not applied between the reflecting pixel electrode 10 and the opposing electrode 14, linearly polarized light having transmitted through a polarizer 21b and having an alignment angle of 90 degrees transmits through a liquid crystal layer 13 and then becomes right circularly polarized light. Thereafter, the right circularly polarized light reaches a reflecting pixel electrode 10 and is reflected as left circularly polarized light by the reflecting pixel electrode 10, and again transmits through the liquid crystal layer 13 and becomes linearly polarized light having an alignment angle of 0 degrees, preventing the light from exiting the device and in turn being followed by a display of black color. When a voltage is applied between the reflecting pixel electrode 10 and the opposing electrode 14, a liquid crystal molecule of the liquid crystal layer 13 vertically rises. In this case, the linearly polarized light having transmitted through the polarizer 21b and having an alignment angle of 90 degrees keeps unchanged even after transmission through the liquid crystal layer 13. Then, the light reaches the reflecting pixel electrode 10 and is reflected by the reflecting pixel electrode 10, and again transmits through the liquid crystal layer 13 and exits the device while keeping its linearly polarized state and having an alignment angle of 90 degrees, leading to a display of white color. That is, the device operates in a normally-black mode in the reflective region 5. Since how the device operates in the transmissive region 6 is the same as that explained in the description of the device shown in FIG. 20(c), the explanation thereof is omitted. However, it can be concluded that the device operates in a normally-white mode in the transmissive region 6.
As noted above, when liquid crystal molecules in the transmissive region 6 are driven by a horizontal electric field and in addition, even when liquid crystal molecules in the reflective region 5 are driven by either a horizontal electric field or a vertical electric field, the device operates in a normally-black mode in the reflective region 5 and operates in a normally-white mode in the transmissive region 6, meaning the device is far from serving as a practical usage. If one tries to force the device to display images, one has to make polarity of an image signal input to the reflective region and polarity of an image signal input to the transmissive region opposite to each other, causing significant difficulty in designing a device structure and processing a signal.